This application claims the priority of German patent document 100 25 667.8, filed May 24, 2000, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method for operating a gas generation device in a fuel cell system.
Gas generation devices for generating hydrogen are started in various ways. For example, in International patent document WO 96/00186 A1 autothermal or partial oxidation reactors are started by means of an exothermic reaction in the interior of the reactors.
Moreover, reactors for steam reforming which are heated by means of an exothermic reaction in a thermally coupled region and/or by the introduction of a hot heat-transfer gas are also known (See, for example, German patent document DE 33 45 958 A1).
However, reactors of this type have a high thermal mass, and therefore need a very long time to be heated to a predetermined operating temperature. To avoid this drawback, a multistage reformer which has at least one subunit with a reduced thermal mass (compared to the other subunits) is known from European patent document EP 0 968 958 A1.
It is an object of the present invention to provide an improved method for operating a gas generation device in a fuel cell system, in particular with improved cold-start properties.
This and other objects and advantages are achieved by the method according to the invention, in which at least two gas generation units, through which a starting-material stream flows in sequence, are provided in a gas generation device, with the first gas generation unit having a lower thermal mass than the second gas generation unit. During a starting phase, only the first gas generation unit is operated, with a power and/or at a temperature which lie above the values encountered in normal operation of the gas generation device. After the end of the starting phase, at least the second gas generation unit is then operated.
Therefore, for the cold start, only a subunit of the overall gas generation device is used. This means the hydrogen immediately after the start is generated in a thermally separate unit, while the remaining hydrogen-generating region is still cold. This drastically reduces the mass which has to be heated.
As a result of the low thermal mass and the overload operation of the first gas generation unit during the starting phase, the cold-start time of the gas generation device is shortened. Particularly in mobile applications in the automotive sector, cold start must take place very quickly. Moreover, the reduced thermal mass reduces the quantity of oxygen required during the starting phase in catalytically heated systems.
The possibility of using the first gas generation unit (after the starting phase) only in full-load operation enhances the service life of the first gas generation unit, which is limited as a result of overload operation during the starting phase. Because this unit is switched off (or running a reduced load) in partial load phases, it is possible overall to provide a fuel cell system of sufficient service life.
The possibility of the first gas generation unit also operating in the partial load range in the event of substantial load changes has the advantage that a sufficient quantity of hydrogen can be provided within a short time, so that it is possible to improve the dynamics of the system. Moreover, the low thermal mass of the first gas generation unit makes it possible to keep this unit warm by suitable means where necessary (i.e., to keep it at a temperature which lies between ambient temperature and operating temperature). In this way, the cold-start time can be shortened further.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.